drm_hwcomposer: Use -Werror in external/drm_hwcomposer

* Suppress non-critical warnings.

I'm submitting this patch to try to align aosp/master
with the upstream fdo/master branch.

Bug: 66996870
Change-Id: I48e87526e408980c90e8a45e134f95da7a89a19d
Signed-off-by: John Stultz <john.stultz@linaro.org>
diff --git a/Android.mk b/Android.mk
index c0a1d0e..8142883 100644
--- a/Android.mk
+++ b/Android.mk
@@ -16,6 +16,15 @@
 
 LOCAL_PATH := $(call my-dir)
 
+common_drm_hwcomposer_cflags := \
+    -Wall \
+    -Werror \
+    -Wno-unused-function \
+    -Wno-unused-label \
+    -Wno-unused-parameter \
+    -Wno-unused-private-field \
+    -Wno-unused-variable \
+
 # =====================
 # libdrmhwc_utils.a
 # =====================
@@ -24,6 +33,8 @@
 LOCAL_SRC_FILES := \
 	worker.cpp
 
+LOCAL_CFLAGS := $(common_drm_hwcomposer_cflags)
+
 LOCAL_MODULE := libdrmhwc_utils
 LOCAL_VENDOR_MODULE := true
 
@@ -71,6 +82,8 @@
 	virtualcompositorworker.cpp \
 	vsyncworker.cpp
 
+LOCAL_CFLAGS := $(common_drm_hwcomposer_cflags)
+
 LOCAL_CPPFLAGS += \
 	-DHWC2_USE_CPP11 \
 	-DHWC2_INCLUDE_STRINGIFICATION